The invention relates to thermoforming and thermoformed containers and in particular to a thermoformed container having an inwardly extending cut lip.
Thermoforming is a process of heating a thermoplastic sheet to working temperature and then forming it to a finished shape by means of heat or pressure. In a typical thermoforming system set up for continuous forming, the sheet is heated in an oven then moved to a forming press. There the softened plastic material is forced against the mold surface by vacuum or air pressure until it sets. During the forming process, a sheet clamp may hold the sheet flat at the edges of the mold. A single mold may be used or matched male and female molds may be used to assist the forming process.
When the material has set, it may be moved to a trim press where a steel rule or matched punch and die cut the part from the sheet. The use of separate dies can cause some misregistration between the molded part and the cutting die, and accordingly a cut in-place configuration may be used where the sheet is formed and trimmed in the same station.
Thermoforming can produce extremely thin-walled parts with low tooling costs. Nevertheless, there are a number of limitations to the thermoforming process. First, the cut lip of the flange may give the products an unfinished look. To the extent that the cut lip reveals the thinness of the material, packaging using thermoforming techniques may look inexpensive.
Even with cut in-place systems, it is difficult to control the dimension and alignment of the cut lip, limiting the application of thermoforming in products where a tight tolerance must be held on this outer dimension.
The light weight and low cost of thermoformed products make it critical that the products be nested for economical shipping. Nevertheless, the implicit symmetry to a thermoformed sheet causes nested products to stack tightly, making de-nesting a problem.
The cut lip of the thermoformed product interferes with attempts to create a closed container with tight sealing.